网上有的说需要如下设置：
setCPLConfigOption("SHAPE_ENCODING","")
我这里没有效果

我用的版本：
rgdal: version: 1.2-11, (SVN revision 676)

解决方案：
poi <- readOGR(dsn, layer, use_iconv = TRUE, encoding = "UTF-8")

只设置encoding是没有用的，因为：
The encoding-parameter is ignored, if iconv is not set to TRUE.

官方文档说明    encoding：

default NULL, if set to a character string, and the driver is “ESRI Shapefile”, and use_iconv is FALSE, it is passed to the CPL Option “SHAPE_ENCODING” immediately before reading the DBF of a shapefile. If use_iconv is TRUE, and encoding is not NULL, it will be used to convert input strings from the given value to the native encoding for the system/platform.